Electric furnaces are commonly used for melting of metals and for smelting and reduction of ores. In such furnaces, heat is typically supplied to the furnace charge through one or more cylindrical electrodes, each of which is vertically suspended through the furnace roof by an electrode column.
The electrode column not only supports the electrode, but is also responsible for carrying electrical power to the electrodes, positioning the electrodes based on furnace power requirements, incrementally feeding or “slipping” the electrodes downward into the furnace as they are consumed and, where the electrodes are of the Soderberg type, helping in the baking of the electrodes. It is also desirable to be able to move the electrode incrementally upward, for example to correct for over-slipping or to compensate for rapid bath rise in the furnace.
In order to perform these functions, a typical electrode column comprises a hoist to control the position of the electrode, an electrode slipping system to incrementally feed the electrode through the electrode column and into the furnace, a paste heater to soften unbaked paste blocks and ensure correct baking of the electrode, a power clamp to deliver electrical power to the electrode, and an electrode seal to prevent excessive release of harmful gases from the opening in the furnace roof through which the electrode extends.
The power clamp typically comprises a number of copper contact pads which are biased against the surface of the electrode to maintain electrical contact with the electrode. This clamping force of the electrode clamp on the electrode through the contact pads is continuously applied, typically only being released for maintenance.
The electrode slipping system typically comprises a slipping clamp assembly having one or two slipping sleeves which apply a radial clamping force to support the electrode. In prior art slipping clamp assemblies, at least one of the slipping sleeves is provided with means to release the clamping force, thereby permitting the electrode to “slip” downward relative to the electrode column.
A description of a typical prior art system for incrementally raising and lowering an electrode is the “two-ring” system described at column 1, lines 20 to 34, of U.S. Pat. No. 4,481,637 (Evensen) issued on Nov. 6, 1984. According to this system, two holder rings are provided which exert radial forces on the electrode. When feeding the electrode downwardly into the furnace, the pressure of the first of the holder rings is released and the first ring is moved vertically relative to the second holder ring and the electrode, following which the pressure on the first holder ring is reactivated. The pressure on the holder ring is then released and the electrode is fed down by lowering the first holder ring. When the feeding movement is finished, the pressure on the second holder ring is reactivated. Therefore, this two-ring system involves releasing the pressure on at least one of the holder rings during feeding of the electrode.
Another prior art electrode slipping mechanism is described in U.S. Pat. No. 4,243,832 (Parsons et al.) issued on Jan. 6, 1981. The Parsons et al. patent describes and illustrates an electrode slipping system in which two sets of girdle clamps 5,6 are arranged inside a support frame 3, the frame 3 being provided with a lever arm 10 having a double acting ram 11. In order to lower the electrode, the ram 11 is extended with the clamps 6 tightened about the electrode. Extension of ram 11 causes lowering of end 13 of lever arm 10, resulting in the electrode being slipped downwardly against the friction of clamps 5. After the electrode is lowered, the clamps 6 are loosened and moved upwardly to their initial positions. In order to raise the electrode, the clamping forces of the lower clamps 6 are released, and the ram 11 is then extended to lower the clamps 6, after which the clamping force of clamps 6 are reactivated. The clamps 6 are then raised in relation to clamps 5, raising the electrode against the frictional force of clamps 5. Although the clamping forces applied to the electrode by the upper set of clamps 5 are sufficient to support the electrode, the Parsons et al. system nevertheless involves release of the lower clamps 6 during raising and lowering of the electrode.
Another prior art electrode slipping system is described in U.S. Pat. No. 3,898,364 (Hardin) issued on Aug. 5, 1975. The Harden patent discloses a system having a contact clamp 14, a slipping clamp 16 and a torquing clamp 18, each of which can be tightened and released. During operation, the electrode is supported by the contact clamp 14 and is lowered by releasing the clamping pressure on clamps 16 and 18 and moving clamp 14 down relative to 16. Thus, the Hardin patent also discloses a system in which the clamping pressure on the electrode is released.
As the weight of a typical electrode is substantial, being on the order of about 40 tons, the release of clamping pressure on the electrode during feeding or slipping is undesirable, as the safety factor against dropping the electrode is reduced when one or more clamps is released. Accordingly, there is a need for improved electrode slipping systems in which release of clamping forces on the electrodes is avoided.